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- The Post Plotting Instrument, or simply Post Instrument, was the standard optical sighting system used by the UK's Royal Observer Corps (ROC) to determine the location of aircraft. It was used during the period from the mid-1930s into the early 1950s, and was one of the main sources of daytime tracking information during World War II. There were two versions of the Post Instrument, a pre-war model using a pantograph, and a wartime version of somewhat more sophistication. Both required the operator to estimate the altitude of the aircraft and enter that into the device, then point a mechanical indicator, or sight, at the aircraft. The motion of the sight moved an indicator on a small Ordnance Survey National Grid map. The grid location indicated by the pointer was then telephoned to central control rooms, where several such reports were combined to produce a more accurate location estimate. Later models added the Micklethwait Height Corrector, which allowed the posts to measure altitude with some accuracy and thus improve the quality of the measurements. The ROC also developed a methodology that allowed the Post Instrument to be used to produce measurements purely by sound, but it is unclear how often this was used. (en)
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